X-RAY-DIFFRACTION STUDY OF CALCIFICATION PROCESSES IN EMBRYOS AND LARVAE OF THE BROODING OYSTER OSTREA-EDULIS

X-ray powder diffraction was used to study shell calcifications of the oyster Ostrea edulis, sampled in the Limski Kanal, Istria (Adriatic S ea), in May 1992. All the developmental stages were followed, from the embryonic stage through the transition between the trochophore and ve liger larva (prodissoconch I and II) and later, after swarming, the pe lagic free-swimming larval stages, up to their settlement and attachme nt (from the D-shaped to the fully formed pediveliger larva), and fina lly during metamorphosis and juvenile stages (dissoconch). In the firs t gastrula stage, only an amorphous tissue is present (a periostracum and organic matrix). The beginning of shell formation (at the end of g astrulation) in early trochophores is manifested by the appearance of calcite (up to 1-7% of total volume) and then aragonite (about 1%). In the later stage of the veliger larva the fraction of calcite decrease s as well as the amorphous fraction, while the fraction of aragonite r apidly increases. In the prodissoconch II stage and during the whole p elagic period aragonite is dominant, accompanied by a very small amorp hous fraction and traces of calcite. The shell mineral composition doe s not change until metamorphosis, whereupon the fraction of calcite ra pidly increases and the fraction of aragonite decreases. The postmetam orphic valves of the juvenile and adult oyster consist mainly of calci te, except the resilium and myostracum which remain aragonitic, possib ly as a continuation of the inner layer of the larval shell.